Process of hot-dip galvanizing and alloying

Abstract

A method of producing a hot-dip galvanized steel strip having a uniform zinc-iron alloy coating on one surface and a formable hot-dip metallic zinc coating on the opposite surface which comprises forming a thin uniform hot-dip metallic zinc coating which has a coating weight less than 0.10 ounces per square foot on one side and a heavier metallic zinc coating on the opposite surface and passing the coated strip through a chamber having heating means and cooling means therein, said heating means, such as gas jets, adapted to apply heat directly only to the thin zinc coated side of the strip to convert the thin zinc coating into a fully alloyed zinc-iron coating, and said cooling means, such as air jets, adapted to blow cooling air onto the heavier zinc coating to effect rapid cooling thereof while the thin zinc coated side is being heated; thereby preventing formation of a thick zinc-iron intermetallic subsurface layer on the heavier coated side.

We claim: 
     
       1. A continuous process for producing a galvanized ferrous metal strip on a continuous hot-dip coating line having a zinc-iron alloy surface coating on one lateral surface thereof comprising, continuously moving an endless ferrous metal strip through a hot-dip coating bath providing a hot-dip zinc coating on both lateral surfaces with one said zinc coating being a light zinc coating having a weight not exceeding about 0.25 ounces per square foot and preferably less than about 0.10 ounces per square foot, heating a surface area of said light zinc coating with heating means disposed in a treating zone and simultaneously cooling the surface of the other zinc coating in an area directly opposite said area being heated while said strip is moving through said treating zone by cooling means disposed in said treating zone, said heating transforming said light zinc coatng into a zinc-iron alloy surface coating and said cooling maintaining said other zinc coating during heating in said treating zone below a temperature at which there is significant increase in the thickness of the subsurface zinc-iron intermetallic alloy layer of said other zinc coating. 
     
     
       2. A continuous coating process as in claim 1, wherein said other zinc coating has a thickness of between about 0.35 ounces per square foot and about 1 ounce per square foot, and the subsurface intermetallic compound formed during said process has a thickness which does not exceed about 10% of the thickness of said other zinc coating. 
     
     
       3. A continuous process as in claim 1, wherein said light zinc coating has a coating weight less than about 0.10 ounces per square foot. 
     
     
       4. A continuous process as in claim 1, wherein said ferrous metal strip has a thickness between about 0.025 and about 0.035 inches. 
     
     
       5. A continuous process for producing a galvanized ferrous metal strip on a continuous hot-dip coating line having a zinc-iron alloy surface coating on one lateral surface thereof comprising, continuously moving an endless ferrous metal strip through a hot-dip coating bath providing a hot-dip zinc coating on both lateral surfaces with one zinc coating being a light zinc coating having a weight not exceeding about 0.25 oz. per square foot and preferably less than about 0.10 oz. per square foot, applying heat to an area of the said light zinc coating in a treating zone while simultaneously cooling the other zinc coating in an area directly opposite the area of the thin zinc coating being heated as said strip is continuously moved through said heating zone, said thin zinc coating being heated in said treating zone to a temperature between about 950° F. and about 1050° F. for a period between about 4 to 6 seconds while said cooling is effected by blowing air at a temperature of about 60° F. onto the said area of the other zinc coating directly opposite the area of the thin zinc coating being heated, and said heat effecting transformation of said light zinc coating into a zinc-iron alloy surface coating and said cooling maintaining said other zinc coating below a temperature at which a significant amount of subsurface zinc-iron intermetallic compound is formed.